The invention concerns a device for producing spherical balls comprising:
means for feeding, in a fusion vessel, material intended for the manufacture of balls,
a second vessel communicating with the fusion vessel in order to receive the melted material,
means to form, from the melted material contained in the second vessel, a jet through at least one orifice,
vibrating means used to transmit vibrations to the orifice, in order to transform the jet into droplets,
a cooling tower, arranged at the outlet of the orifice and filled with an inert gas, wherein the droplets, falling by gravity, solidify to form the balls, and means for receiving balls at a lower end of the cooling tower
There exist numerous methods and granulation devices, applied in numerous industrial domains, for example in metallurgy, fertilisers, food or pharmaceutical industry, etc . . . They all aim at transforming into balls materials which, melted, have, in open air or under neutral gas, low viscosity, good surface tension, good flowability through orifices and are liable to solidify by cooling.
For exemplification purposes, the document WO-A-8101811, corresponding to the patent U.S. Pat. No. 4,428,894, describes a method and a device for producing metallic granulates, 0.1 to 5 mm in diameter, from a melted metal bath. According to this known method, a jet of melted metal is formed, it is passed through a vibrating orifice in order to divide the jet into individual drops, the drops are caused to fall from the jet by gravity through an atmosphere of inert gas, in order to cause the drops to solidify by cooling into granulates.
However, in practice, there are few methods and calibrated granulation devices, i.e. capable of large-scale producing, with good output, balls of uniform sizes with very good surface condition. Besides, whatever the material (organic product, metal . . . ) granulated by this type of technique, it often happens that a certain percentage of the production is composed of balls vastly deformed or of balls stuck together, often in pairs, such a pair being designated as a dumbbell.
Still, it is necessary, for certain applications, to produce balls, more specifically microballs, without any surface defects and with very accurate granulometry. These microballs must, preferably, not only have uniform diameter, but also be perfectly spherical and isolated. In the mechanical industry, when balls are composed of a material with relatively high hardness, one uses generally selection devices or sorting devices, comprising hoppers, rolls, etc . . . It is the case notably of balls for bearings, for pens, etc . . . The balls may then sustain shocks and frictions without marking their surface. Conversely, in certain cases, the sphericity and the surface condition of the balls should not be altered by the selection device.
It is the case, notably, for the welding alloyed balls intended to be used in electronics to form connections, for example for casing of BGA ( less than  less than Ball Grid Array greater than  greater than ) type. Still, the metals used in the welding alloys are soft metals and the surface condition of a ball composed of such metals can be altered very easily.
The purpose of the invention is to obtain balls without any surface defects. More particularly, the balls must be perfectly spherical, with excellent surface condition, without any oxidisation. The composition of the balls must be very stable with each batch of manufacture and the tolerances very narrow as regards granulometry.
According to the invention, this target is met in that the device comprises an outlet chamber in the fusion vessel and means for ultrasound stirring of the melted material contained in the outlet chamber before its transfer into the second vessel, whereas the inert gas in the cooling tower contains a pre-set quantity of oxygen.
According to a development of the invention, the means for receiving balls comprise shock-absorbing means. Said means comprises, preferably, brushes composed of polyamide-based wired, forming an angle of approx. 45xc2x0 with the trajectory of the balls in the cooling tower. Cloth rollers can, moreover, be arranged on the inner periphery of the inner wall of the cooling tower, above the brushes.
According to a preferred embodiment, the device comprises, at the outlet of the cooling tower, means for periodic withdrawal of the balls and calibration means comprising means for sorting the balls into three categories, in relation to their sizes. It may also comprise means for weighing the set of balls of each category obtained at each withdrawal sequence, means for determining, on the basis of information supplied by the weighing means, a percentage of balls within pre-set standards and means for adjusting the frequency of the vibrating means in relation to said percentage.
It is another object of the invention to provide a device for producing spherical balls with a selection device of spherical balls enabling to sort balls of uniform diameter, in order to put aside all the balls which are not perfectly spherical.
According to the invention, this target is met by the fact that the selection device comprises means for supplying the balls on a first tilted plane of a succession of tilted planes, separated by spaces of pre-set dimensions, whereby at least the first tilted plane, in the direction of displacement of the balls, has a surface whereof the roughness is greater than that of the following tilted planes.
According to a preferred embodiment, the means for supplying the balls on the first tilted plane comprise a rolling plane, stop means, ejection means to eject from the rolling plane, jerkily, the balls resting against the stop means.
The ejection means comprise, preferably, a hollow ramp, animated by a lateral back and forth movement and fitted with a plurality of air ejectors distributed evenly along the ramp.
According to another characteristic of the invention, the means for supplying the balls on the first tilted plane comprise a conveyor belt having a tilted face in the same direction as the tilted planes and a rotational direction opposite the rolling direction of the balls on said face.
Another object of the invention concern the elimination of the balls stuck together.
According to a development of the invention, this other object is met by the fact that the selection device comprises, downstream of the tilted planes, a toothed wheel animated by a rotational movement round a tilted axis and comprising longitudinal grooves whereof the sizes enable an isolated ball to roll and prevent a pair of stuck balls from rolling.